Optical nanoantennas based on resonant plasmonic elements have been extensively studied for enhancement of luminescence of localized light sources (such as fluorophores, dyes, quantum dots etc.) in the visible and infrared (IR) spectral ranges. Optical nanoantennas, similar to their ancestors, microwave antennas, can efficiently convert propagating radiation to localized energy, and vice versa, but in the optical frequency range. Owing to the resonant nature of plasmonic elements, plasmonic antennas can create high near-field enhancement localized around the antenna and enhance the far-field luminescence of localized light sources. The main disadvantage of plasmonic nanoantennas, similar to all plasmonic-based devices, is strong losses of the plasmonic materials in the optical spectral range. Due to these losses, most of the energy tends to dissipate, heating the antennas rather than emitting it into the far-field.
Recently a concept of optical nanoantennas based on resonant dielectric nanostructures has been proposed and theoretically investigated. Similar to plasmonic elements, high-refractive index dielectric particles may possess strong resonances at optical frequencies. However, hi contrast to plasmonics, these resonances are associated only with displacement (polarization) currents with no real currents and thus no Ohmic losses involved. Thus, in transparent dielectric materials, this resonant behaviour can be practically loss-free. It has been shown theoretically that high-refractive index dielectric antennas can provide near-field enhancement and fluorescence enhancement of electric-dipole emitters comparable to plasmonic antennas. They can also significantly over-perform plasmonic antennas when an electric dipole emitter is located relatively far from the antenna surface (>50 nm) or a magnetic dipole emitter is used to excite the antenna. Performance of dielectric antenna designs for directivity and field enhancement has been verified experimentally in the GHz spectral range when all the material parameters were chosen to simulate a silicon nanoantenna at the visible and near-IR frequencies.
Until now, dielectric nanoantennas have only been designed to enhance the luminescence of point dipole sources in directions parallel to the nanoantenna plane (e.g., plane of the substrate at which the antenna can be fabricated).